1. Field of the Invention
This invention relates to a method for preparing a mixture of short-chain polysulfide silanes for use in silica-loaded tire compounds.
2. Prior Art
In prior art silica-loaded tire compounds, bis(triethoxysilylpropyl)tetrasulfide is often used as a coupling agent for forming a bond between rubber and silica. This compound, however, has the problem that when milled with rubber and silica at elevated temperatures, it causes the blend to increase its Mooney viscosity to such an extent as to restrain subsequent working.
Then JP-A 259739/1996 discloses a method for improving the workability of a rubber compound loaded with silica and bis(triethoxysilylpropyl)tetrasulfide. When bis(triethoxysilylpropyl)tetrasulfide is used alone, however, workability is improved at the sacrifice of low fuel consumption characteristic of the silica-loaded tires.
We thus paid attention to polysulfide silane mixtures and discovered that a good compromise is made between low fuel consumption and workability when a mixture of polysulfide silanes represented by the following general formula (1) is used. EQU (RO).sub.3 SiC.sub.3 H.sub.6 S.sub.n C.sub.3 H.sub.6 Si(OR).sub.3(1)
Herein n is a positive number having a distribution whose average is in the range: 2.2.ltoreq.n.ltoreq.2.8. and R is methyl or ethyl.
With respect to the preparation of a mixture of polysulfide silanes of formula (1), JP-A 228588/1995 discloses a method involving reacting Na.sub.2 S with sulfur to produce polysulfides and continuously reacting the reaction product with a halogenoalkoxysilane without isolating the polysulfides.
This preparation method, however, has some problems. Because of a substantial variation between lots, the distribution of n in formula (1) sometimes deviates from the desired range. The reaction with the halogenoalkoxysilane is effected in a large amount of solvent, which leads to a low pot yield.
With respect to reaction of sodium polysulfide with halogenoalkoxysilane, JP-A 29726/1973 discloses reaction between Na.sub.2 S.sub.4 or Na.sub.2 S.sub.2 alone and a halogenoalkoxysilane. This patent pertains to the reaction of a single compound, sodium tetrasulfide or sodium disulfide, but teaches nowhere the reaction of polysulfides having a distribution (2.2.ltoreq.n.ltoreq.2.8) with a halogenoalkoxysilane.
Therefore, an object of the invention is to provide a novel and improved method for preparing a mixture of short-chain polysulfides of formula (1) in a simple, efficient and steady manner in high yields.